The present invention relates to a method of and an apparatus for detecting contaminating particles (a foreign substance) on a reticle (or photo-mask), before a circuit pattern on the reticle is projected on a wafer (or substrate) to form the circuit pattern thereon.
Conventional apparatuses for detecting a foreign substance on a reticle or photo-mask are disclosed in JP-A-59-65,428 and JP-A-54-101,390.
JA-P-59-65,428 discloses a foreign substance detecting apparatus which includes a laser device for emitting a linearly-polarized laser beam, means for causing the laser beam to impinge upon a reticle at a specified incident angle, and a focusing optical system made of a polarizer and a lens. In this apparatus, only reflected light from a foreign substance on the reticle is taken out by utilizing a fact that when the reticle is irradiated with the linearly-polarized light, the reflected light from the substrate and circuit pattern of the reticle is different in state of polarization from the reflected light from the foreign substance, and thus the foreign substance can be detected.
JP-A-54-101,390 discloses a foreign substance detecting apparatus which includes a laser device for emitting a laser beam, means for irradiating a specimen obliquely with the laser beam, a Fourier transform optical system formed of a lens, a spatial filter disposed in a Fourier transform plane, and a focusing optical system. In this apparatus, based on a fact that the greater part of a circuit pattern in a field of view is usually made up of pattern portions extended in one or a few directions, the diffracted light from the pattern portions is eliminated by the spatial filter disposed in the Fourier transform plane, to emphasize and take out only the reflected light from a foreign substance on the specimen.
Further, matters concerned with the foreign substance detecting apparatuses are described in, for example, a Japanese publication entitled "OYO-KOGAKU (Applied Optics)" by H. Kubota, pages 129 to 136 published by Iwanami Shoten, Publishers.
Recently, the integration density of an LSI (namely, large scale integration circuit) has been enhanced and the wiring pattern of the LSI has been made very fine. Thus, a fine foreign substance has become a serious problem. Further, a filmy foreign substance is formed of the remainder of a resist film in fabricating a reticle, the residue of chromium or chromium oxide for forming a pattern, or the condensate of impurities dissolved in a cleaning solution for a reticle. The above filmy foreign substance also has become a serious problem.
It is impossible for the techniques disclosed in JP-A-54-101,390 to discriminate a fine massive (or lump) foreign substance and a filmy foreign substance from a circuit pattern, since the reflective light from these foreign substances is very weak. The fine massive foreign substance can be emphasized by the techniques disclosed in JP-A-59-65,428, but the number of eliminable circuit patterns is limited. That is, it is impossible to eliminate all the circuit patterns by the same spatial filter.